Electrooptical transmission



Oct. 13, 1931. H. NYQU IST ELE CTROOPTICAL TRANSMISSION Filed May 26 1927 2 Sheets-Sheet 1 BY Arrow/5y Oct. 13, 1931.

H. NYQUlS T ELECTROOPTICAL ITRANSMI S S ION Filed May 26, 1927 2 Sheets-Sheet 2 Patented Oct. 13, 1931 UNITED STATES PATENT OFFICE HARRY NYQUIST, OF MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AN D TELEGRAPH COMPANY, A CORPORATION OF NEW YORK ELECTROOPTICAL TRANSMISSION Application filed May 26, 1927.

This invention relates to translating variable energy of one form into variable energy of another form, and more particularl to the translation of electric energy into hght of varying intensity for producing an image or picture at the receiver of a picture transmlssion system.

Heretofore, it has been customary in the electrical transmission of pictures to transmitpicture currents, corresponding to the tone values of a picture, from a sending station to a receiving station, a light valve being actuated by the received picture currents to control the amount of light, to which a sensitive film is exposed. A more complete dis closure of a plcture transmission system of this type may be found in Patent No. 1,606 227, issued to Horton, Ives and Long, November 9, 1926.

A specific embodiment of the presentinvention, herein chosen for purpose of illustration, comprises two plane mirrors having their planes intersecting at right angles and controlled by incoming picture current at the 5 receiving station, which mirrors take the place of the light valve used in the system above described. The term 90 degree mirror is used hereinafter to designate such an arrangement of plane mirrors. This 90 degree mirror rotates about an axis at the line of intersection. The surfaces consist of alternately reflecting and non-reflecting strips of gradually increasing width from the line of intersection. The rotation of the 90 degree mirror is controlled jointly by picture currents received from a transmission line, which currents pass through a movable coil attached to the 90 degree mirror, and by current from a local source which passes through 40 a stationary coil. the position of the 90 degree mirror varying in accordance with the amount of current received from the line. A constant light source is arranged to project a beam of substantially parallel rays of light toward the surfaces, the axis of the beam be- Serial No. 194,381.

ing directed toward the axis, or intersection line, of the surfaces and at an angle thereto. The reflected beam from these surfaces is directed to a focal point on a light sensitive surface, such as a photographic film. The amount of light reflected by the surfaces and, therefore, the intensity of the light at the focal point will vary directly with the angular change in position of the surfaces as con trolled by the picture currents received from the sending station. The reflecting strips on the surfaces may be so designed as to give a non-linear relation between the light intensity and the received current strength.

The invention will be more fully explained in the following detailed description having reference to the accompanying drawings.

Figures 1, 2 and 3 are various diagrams used for explaining the principles on which the invention is based;

Fig. 4 is a perspective view of one embodiment of the invention in a receiver of an electro-optical system illustrating an arrangement of the 90 degree mirror with respect to an incident beam from a light source and a reflected beam directed to a sensitive surface, the axis of the incident beam being at an angle of inclination less than 90 degrees to the axis of the 90 degree mirror;

Fig. 5 is a diagrammatic plan view of Fig. 4 showing the intensity of'the reflected light obtained when the 90 degree mirror is in normal position,

Fig. 6 shows the change in intensity of reflected light when the 90 degree mirror is moved to another angular position with respect to the light source;

Fig. 7 shows diagrammatically a modified arrangement of the invention in which a semitransparent mirror is positioned at an angle of 45 degrees with respect to the axes of the incident beam and the reflected beam;

Fig. 8 is an enlarged diagram of the 90 degree mirror used in explaining the construction of a mirror in accordance with this invention;

Fig. 9 is a perspective view of a 90 degree mirror embodying this invention with alternate reflecting and non-reflecting parallel strips on the surfaces; and

Figs 10a, 10b and 100, respectively, show side, front and plan views of a modified arrangement of a combined lens and 90 degree mirror according to this invention.

The principles on which the invention is based may be better understood by referring to Figs. 1, 2 and 3. In Fi 1 there are represented two plane mirrors and ON perpendicular to the plane of the drawings and intersecting at right angles as shown at O. A ray of light EF is projected on the mirror OM and reflected from point F on the mirror OM to point G on the mirror ON, the reflected ray being again reflected to a point H. Considering the two mirrors as a unit, EF may be called the incident ray and GH the reflected ray. Ihese rays are parallel and if a line O-O is drawn from the intersection O of the mirror unit parallel to the rays EF and GH, it will be seen that w=w where w and o are perpendicular to 0-0. It, therefore, follows that if the mirror unit is rotated about an axis through O, the line of intersection of the mirror surfaces, to the position OM and ON so that the incident ray strikes the mirror OM at point F and is reflected to point G on the mirror ON, then the second reflected ray GH coincides with the first reflected ray OK, i. e., the reflected ray is unaltered in location and direction by the rotation of the mirror unit, provided the angle between the surfaces OM and ON is maintained at 90 degrees. Consequently, if a beam of light is so directed at the mirror unit that the axis of the beam passes through the axis O, the beam will be reflected without inversion and the axis of the reflected beam will also pass through the intersection O, regardless of the angular position of the 90 degree mirror. The effect of the 90 degree mirror is to reverse exactly the direction of the beam.

In Fig. 2, the plane surfaces OM and ON are altered so that only the strips (1 on the plane OM and a on the plane ON reflect light. These strips are equal in size and equi-distant from the axis O. Now if a beam of light I is directed to the 90 degree mirror and the mirror is rotated to a position so that the angle of incidence of the beam is 4.3 degrees (this position will be called the normal position), that portion of the beam which falls on (1 will be completely reflected from a and vice versa. If the axis of the beam is rotated through an angle a, which is the same as rotating themirror through the same angle, only a portion of the beam directed to the reflecting strip a on the plane OM will be reflected to the reflecting strip a on the plane ON, as indicated by c. The portion 0 will again be reflected parallel to the axis of the incident beam. The portion 0 falling outside of a will not be reflected. In a similar manner a portion of the incident beam reflected from (1 which is totally reflected by a with the 90 degree mirror in normal position will new fall outside of a and not be reflecte It is evident, then, that the amount of light reflected by the reflecting strips a and a depends on the position of the 90 degree mirror with respect to the light source and is less the greater the value of a up to the point where no light is reflected.

In Fig. 3, the conditions are varied by making the portion a on the plane ON nonreflecting and providing reflecting strips (1 and a on either side of the non-reflecting strip a In this case, the incident beam I directed to the reflecting strip a, on the plane OM is not reflected when the 90 degree mirror is in normal position, i. e., when the 90 degree mirror is at an angle of 45 degrees with respect to the axis of the light beam. However, if the axis of the beam is rotated through an angle a as described in connection with F i 2, a portion of the incident beam reflected y a will fall on a as indicated by d, and be reflected and the portion d will not be reflected. Likewise, a part of the incident beam reflected by a, will fall on a and be reflected. It is obvious, then, that in this case the amount of light reflected by the reflecting strips will also depend on the position of the 90 degree mirrors but here will be greater the greater the value of oz, varying from zero to a point where all the incident light directed to a and a will be reflected.

The arrangement of a 90 degree mirror in a receiver of an electro-optical system is shown in Fig. 4, in which a pair of rectangular plane surfaces 10, are arranged at right angles to each other and are attached to a common shaft 11 at the line of intersection. These surfaces are provided with alternate reflecting and non-reflecting strips. The shaft is mounted between jeweled bearings 12 and 13 to form an assembly which can be rotated about its axis. T he movement of the 90 degree mirror is controlled by current from a source of picture modulated carrier current 40 transmitted from a sending station which passes through a movable coil comprising a winding 14 surrounding an armature 15 attached to the shaft 11 intermediate the bearing 13 and the rectangular surfaces 10. A system for producing picture modulated carrier current is disclosed in detail in the patent to Horton, Ives, and Long referred to hereinbefore. A source of local carrier current 41 exactly synchronized with the carrier component of the current received from the sending station is passed through a fixed coil com rising a winding 16 surrounding an arm 0 the core 17 which is provided with inwardly projecting legs incident and reflected beams.

positioned at either side of the armature 15. 4 The arrangement of the movablelcoil and the I fixed coil is similar to thatemployed in the ordinary dynamometer. A source of light 18 is placed at the principal focus of a projecting lens 19 which is used to producea beam of light having its axis directed to the axis, or intersection line, 'of the 90 degree mirror at an angle of inclination less than 90 degrees, with respect to the vertical axis of the 90 degree mirror, so that the reflected beam from the 90 degree mirror is directed to a focal point, through alens 20, on a. sensitive surfacesuch as a photographic film surrounding a rotatable. carrier 21. The axis of the reflected-beam is at an opposite angle of inclination with respect to the axis of the incident beam. The angle of inclination of the incident and reflected beams should, however, be no smaller than is necessary to obtain the desired separation of the Furthermore, in this simple arrangement of the light. source with respect to the 90 degree mirror, it is desirable to have the axes of the incident and reflected beams in the same vertical plane. This is more clearly shown in Figs.

5 and6 in which Fig. 5-indicates the maximum value of the intensity of the reflected beam in dotted lines when the 90 degree mirror is in normal position; and Fig. 6 indicates a reflected beam of lesser intensity, due to the rotation of the 90 degree mirror so that the incident rays of light are only partially reflected. r It'is evident that rotation of the 90 degree mirror will cause no-dis placement of the reflected beam in a hori} zontal plane and since the path of the ray remains constant in length for all positions of the 90 degree mirror, there will be'no displacement of the reflected beam in a vertical plane. Thus," the reflected beam remains stationary for all the angular positions of the 90 degree mirror with respect to the constant light source, but the intensity of the reflected beam varies with the defle'c tion of the 90 degree mirror A modified arrangement of the light source .with respect to the movable 90 degree mirror is shown in Fig. 7 in which a source of light 22 is placed at the principal focus of aprojecting lens 23 which is used to produce a beam as shown with its axis in the plane of the drawings. This beam encounters a halfsilvered or semi-transparent mirror 24 arranged at an angle of 45 degrees. Part of the beam directed to the mirror 24 passes through the mirror and is lost and the remainder of the beam is reflected to the 90 degree mirror with the axis of the incident beam directed at the axis, or intersection line, of the 90 degree mirror. The light reflected amount of light reflected by a AO' to a focus at a point on the surface 26. The

amount of light reflected by'the 90 degree mirror 10 and, therefore, the intensity of the light'at the focal point on the surface 26 will Vary in accordance with the, position of the 90 degree mirror 10 as controlled by the received current, the same as hereinbefore described in connection with Fig. 4'.

This invention will find embodiment in a great variety of' forms. Probably the most usual form will be that in which the change in intensity of the reflected light from the normal intensity will vary directly with the angular change in position of the 90 degree mirror from its normal position. For this to be so. certain conditions must exist and these will now be determined having special reference to Fig. 8. In this figure strips a and 0 are two reflecting surfaces, each having the width a.

A beam of light is projected on the surfaces MON of suflicient width to cover the surfaces. It is assumed that the amount of light incident to the strips a and a is the same for 9 all positions of the surfaces MON. This is approximately true only when the deflection of the surfaces from their normal position is small. b is the distance of a and (1 from the axis 0 measured to the inner edge of the reflecting surfaces. In the normal position of .the surfaces MON all the incident light re- Now 00=b tan (2 a) and 2b tan oz 1 tan a where v K: tan a 1 +.tan a Similag hy, Therefore a 2 (a b)K amount of light incident @6711 CO a-2 (c+ b)K+ 26K In like manner, the incident light reflected from a to the surface OM falls on DD instead of a and only that portion of this beam which falls on a is reflected for use. In this case where pairs of such strips and as before for the greatest number of strips Without intertan 0: ference K 1 tan 0: e a b amount of light reflected by (1 a-2bK amount of light incident to a a2bK+2(a+b)K' a+2aK'2(a+b)K 1 2(a+b)K v a 2aK a 2aK R +R intensity of reflected beam bK (a+b)K' 2 normal intensity of reflected beam n a2aK a 20K and and if ais small compared with b, e=a (apchange in intensity bK (a+ b)K' p y) lnpthls @356 normal intensity a 2aK a 2aK aR1+eR2 R1 R2 Assuming that the rotation of the 90 degree W mirror from the normal position is sufliciently small so that tan a=a then and I a d a irie sitgf reflected beam bK (a+b)K' 1 a an 1 a intensity of incident beam a 20K (1 2aK from which I Assuming as before that tan a=a,

change in intensity b b) a i change in intensity ba (0 (1)02 normal intensity a(1a) c(1+a) This has been shown to be true for one. pair i. e., when a ven For this case, the width of the strip is From this,

and it is evident that the width of any strip is proportional to the distance of the outside edge of the strip from the point 0.

Considering now the case in which no light is reflected for the normal position of the mirrors, a, and e are reflecting surfaces and a a is non-reflecting. Using the same notation as before, it can be shown that amount of light reflected by e 2bK P amount of light incident to a, a- 211K intensity of incident light a(1 a) a(1 a) This is the same result as obtained for the preceding case.

In either case, then, provided tan oa=a, the change in intensity of the reflected beam is proportional to the quantity ba (a b) a a(1 a) a(1 +04) 01(2b a aa) a (1 a2) For this quantity to vary as a,

a must be smalllcompared to 1, and i au must be small compared to 2b+a.

As a has already been assumed small, these conditions will ordinarily be satisfied. If we assume further that a is small compared with b, the conditions are more easil satisfied. Therefore, the device describe above will give a linear variation in intensity with deflection if the deflection is always small;

It has been assumed above that tan a=a; i. e., a can not be much greater than 3 for maximum deflection. For maximum deflection and a=3,

2b tan 3 1 tan 3 i. e., the width of a strip should not be greater than one-tenth of its distance from the apex 0. However, it will be better to use a narrower strip than this, say, one the width of which is onethirtieth or possibly one onehundredth of its distance from the apex. The strips should not, of course,'be made so narrow that interference effects will result.

Fig. 9 illustrates one form of 90 degree mir ror mounted on a shaft 11 in which each surface 10 is provided with alternate parallel reflecting strips 27 and non-reflecting strips 28 of width gradually decreasing from the outer edge of the surfaces to the axis or intersection line of the 90 degree mirror in accordance with the foregoing formulae. A portion 29 of each surface adjacent to the axis is not used for reflecting purposes due to the effects of light interference and manufacturing limitations. It is understood that the edges of the reflecting and non-reflecting strips on the plane surfaces 10 are parallel to each other and the conditions have been found for a linear variation of light in accordance with the deflection of the 90 degree mirror. In order to compensate for any nonlinearity in other arts of the electro-optical sideration.

' Ives and M. B. Long, November 13, 1928, the,

In a telephotographic system such as is disclosed in Patent No. 1,691,071, to H. E.

operation is such that zero light intensity is received on the film when the line current is a minimum but not necessarily zero. For all values of line current from zero to this allowable minimum, the light intensity on the receiving film remains zero. If the 90 degree mirror of this invention is to be used in such a system the type of mirror should be that WhlCh ivesgzero light intensity for the normal position of the 90 degree mirror; and in order that the intensity will remain zero until the line current exceeds some value, the reflecting strips should be made narrower and the non-reflecting strips wider than those specified above. The amount of change in width of the strips depends on the value assumed for the line current beyond which the light intensity will increase with the current.

The manner of making the surfaces consisting of alternately reflecting and non-reflecting strips may beaccomplished by a method similar to that used for diffraction gratings by ruling lines or strips on speculum metal or plate glass. However, for the proper operation of the 90 degree mirror, it is essential that the corresponding strips on the surfaces OM and ON be exact duplicates of each other. Instead of making these surfaces independently, it is recommended that thesecond surface be laid out photographically from the first surface using a process similar'to that emplo ed in photoeugraving. After one surface has een laid out with alternate reflecting and. non-reflecting parallel strips of graduated widths, the second surface is covered with gelatin mixed with potassium bichromate, or some similar mixture, and then mounted at 90 degrees inclination. By directing a beam of light on the completed surface, the second or prepared surface will be exposed to the light reflected from the first. The unexposed gelatin can then be washed off, the exposed material being insoluble, and the surfaces etched in the usual manner. The second surface will be an exact duplicate of the first surface so that the maximum reflected intensity of light is obtainable when the 90 degree mirror is in normal position.

An alternative arrangement of the rotatable 90 degree mirror and lens system is shown in Figs. 10a, 10b and 100. In this arrangement, the 90 degree mirror is formed by the narrow sides of a 90 degree prism 30 on the face of which is a. lens 31 for projecting the incident beam and focusing the reflected beam. As shown in the side view of Fig. 10a, the axes of the incident beam from S and the reflected beam to S are at an angle of inclination less than 90 degrees with respect to the axis 0 of the prism 30. The outer sur-' faces OM and ON of the prism 30 may be etched in a manner similar to that described .above to form the alternate reflecting and non-reflecting strips on the surfaces. Canada balsam or frosting may also be used to produce the non-reflecting strips on the surfaces. The results obtained with this structure are the same as described hereinbefore in connection with Fig. 4, in which, change in intensity of reflected light varies with the change in angular position of the 90 degree mirror with respect to the light source.

This invention has been described above with special reference to its application to picture transmission. It is also applicable to other electro-optical systems such a television, and in general to any system in which variable energy such as variable electric or sound waves, for example, are utilized to vary a beam of radiant energy.

What is claimed is:

1. In combination, a source ofvarying energy, a source of radiant energy and means to vary the intensity'of a beam of radiant energy from said source in accordance with the variations of said varying energy which means comprises a plurality of elements for successively reflecting said beam, the surfaces of said elements being in fixed angular relation, and means for moving said elements 1n accordance with said energy variations.

2. A. combination in accordance with claim 1 in which said reflecting surfaces are plane.-

3. A device for utilizing variable energy to vary a beam of radiant energy in accordance with claim 1 in which each of said elements has a plurality of reflection surfaces separated by a non-reflecting medium.

4. An electro-optical system comprising means for producing a beam. of li ht, a source of varying current and means or utilizing said electric current variations to vary the intensity of said light beam which means comprises a plurality of elements for successively reflecting said beam, the surfaces of said e-le-' ments being in fixed angular relation, and means for moving said elements in accordance with said current variations.

5. In combination, a source of'radiant energy, and means for varying the intensities of radiant impulses from said source, which comprises a pair of planary elements arranged in intersecting planes and rotatable about a common axis, each of said elements havin a phiralit of reflecting surfaces separated y non-re ecting surfaces and being positioned to reflect energy from said source to the other of said elements.

6. In an electro-optical system, a source of light, a source of va ing electrical energy, and means for modifying light from said source in accordance with the variations of the electrical energy from said source, which means comprises a'pair of planary elements arranged in intersecting planes and rotatable about a common axis each of said elements havin alternate reflecting and non-reflecting sur aces.

7. A device'fortranslating electrical impulses into light impulses of varying intensities, which comprises a pair of planary elements arranged in intersecting planes and rotatable about a common axis, each of said elements having reflecting surfaces separated by non-reflectin surfaces, said reflecting surfaces being 0 adually increasing width from the intersection line of said intersecting planes.

8. In combination, a source of light, and means for varying the intensity of a beam of light from said source, which means comprises a pair of elements, one element being positioned in a plane 90 degrees from the plane of the other element, said pair of ele- .ments being rotatable on a common axis, and

each element having a plurality of separated reflecting surfaces and so positioned that light from said source is reflected thereby to the other of said elements.

9. A device for varying the intensity of light, which comprises a pair of reflecting surfaces located in planes intersecting at right angles, a lens ositioned across the outer edges of said sur aces, and means to rotate said surfaces and lens about the intersection of said planes as an axis.

10. In an optical system for varying the intensity of light waves, a pair of surfaces arranged in intersecting planes at right angles to each other and rotatable about a common axis, a constant light source projecting a beam of light on said surfaces, means'on said surfaces for varying the intensity of the reflected light from said constant light source in proportion to the osition of said surfaces with respect to sai light source, and variable pulsating means for controlling the rotation of said surfaces.

11. In an optical system for varying the intensity of light waves, a pair of surfaces arranged in intersecting planes and rotatable about a common axis, a constant light source projectin a beam oflight rays on said surfaces, re ecting strips of varying width on said surfaces parallel with said axis, said beam of light being projected on said stri s of either surface and being reflected there rom, and means for changing the'position of said surfaces with respect to said 1i ht source, whereby the intensity of the re ected light from said surfaces is directly proportional to the angular position of said reflecting strips with respect to'the axis of said beam of constant light.

12. In an electrical picture transmission system, an oscillatory element having surfaces intersecting at right angles, said surfaces having symmetrical reflecting and nonreflecting strips parallel to the axis thereof, means for producing a'light beam of constant intensity having its axis directed tot-he axis of said element, said reflecting strips in normal position producing maximum reflected light from saidilight beam, and means for controlling the oscillations of said element, said means being energized by current waves. whereby variations in reflected light'are produced in accordance with various positions of said strips with respect to said light beam.

13. In an optical system for varying the intensity of light waves, an element having plane surfaces intersecting at right angles and rotatable about a. common axis, a source of light projecting a constant beam on said surfaces, means on said surfaces for causing a reflected beam to be projected to an objective, the axes of said beams being at divergent angles less thanQO'degrees and inclined to the intersection of said surfaces, and means for causing the rotation of said surfaces to vary the intensity of said reflected beam.

14. In an optical system for varying the intensity of light waves, an element having plane surfaces intersecting at right angles and rotatable about a common axis, a source of constant light, a beam of incident light from said source being projected on said surfaces, means on said surfaces for causing a reflected beam to be rojected toan objective, semi-transparent re ecting means in the path of said incident and reflected beams, and means for causing the rotation of said surfaces to vary the intensity of said reflected beam passing through said semi-transparent reflecting means.

15. The method of changing a beam of radiant energy of constant intensity to a beam of varying intensity by means of a 90 degree mirror having multiple reflecting portions, which method comprises projecting said beam upon said mirror, and changing the angular osition of said mirror with respect to said eam so that the intensity of the reflected beam from said mirror is a function of the instantaneous position of said mirror.

16. The method of changing a light beam of constant intensity to a light beam of varying intensity by means of a combination of alternate reflecting and' non-reflecting surfaces intersecting at right angles, which method com rises projecting said beam to the intersectlng was of said surfaces, and vibrating said surfaces so that the incident light projected on some of the reflecting surfaces is reflected to the opposite reflecting surfaces and the resultant reflected light is proportional to the position of said surfaces with respect to saidconstant light beam.

17. In combination, a source of si aling current, a source of radiant energy an means to vary the intensity of a beam of radiant energy from aid" source in accordance with the variations of the signaling current from said source of signaling current, which means comprises two elements having plane surfaces intersecting at right angles for successively reflecting said beam of radiant energy and means for moving said elements in accordance with the variations of said signal- May, 1927.

HARRY HYQUIST. 

